U.S. patent application number 16/461739 was filed with the patent office on 2019-10-17 for method for producing metal powder.
The applicant listed for this patent is SHOEI CHEMICAL INC.. Invention is credited to Mineto Iwasaki, Tetsuya Kimura.
Application Number | 20190314893 16/461739 |
Document ID | / |
Family ID | 62145744 |
Filed Date | 2019-10-17 |
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United States Patent
Application |
20190314893 |
Kind Code |
A1 |
Iwasaki; Mineto ; et
al. |
October 17, 2019 |
METHOD FOR PRODUCING METAL POWDER
Abstract
A method for producing a metal powder provided on the surface
thereof with a glassy thin film, wherein a glassy substance is
produced in the vicinity of the surface of the metal powder by
spray pyrolysis from a solution that contains a thermally
decomposable metal compound and a glass precursor that produces a
glassy substance that does not form a solid solution with the metal
produced from the metal compound by thermal decomposition, so as to
form the metal powder provided on the surface thereof with the
glassy thin film. The glass precursor is prepared such that the
melting temperature Tm.sub.M of the metal and the liquid phase
temperature Tm.sub.G of the mixed oxide of the glassy substance
satisfy the following formula (1): -100 [.degree.
C.].ltoreq.(Tm.sub.M-Tm.sub.G).ltoreq.500 [.degree. C.] (1).
Inventors: |
Iwasaki; Mineto; (Tosu-shi,
JP) ; Kimura; Tetsuya; (Tosu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SHOEI CHEMICAL INC. |
Shinjuku-ku, Tokyo |
|
JP |
|
|
Family ID: |
62145744 |
Appl. No.: |
16/461739 |
Filed: |
November 9, 2017 |
PCT Filed: |
November 9, 2017 |
PCT NO: |
PCT/JP2017/040351 |
371 Date: |
May 16, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 2301/35 20130101;
B22F 1/0048 20130101; B22F 9/026 20130101; B22F 2302/256 20130101;
B22F 2998/10 20130101; C22C 33/0285 20130101; B22F 2302/25
20130101; B22F 9/30 20130101; B22F 2999/00 20130101; B22F 1/02
20130101; C22C 1/0433 20130101; B22F 2301/15 20130101; B22F 2998/10
20130101; B22F 9/026 20130101; B22F 9/30 20130101; B22F 2201/01
20130101; B22F 1/02 20130101; B22F 2998/10 20130101; B22F 9/026
20130101; B22F 9/30 20130101; B22F 2999/00 20130101; B22F 9/30
20130101; B22F 2201/01 20130101 |
International
Class: |
B22F 1/02 20060101
B22F001/02; B22F 9/30 20060101 B22F009/30 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2016 |
JP |
2016-223134 |
Claims
1. A method for producing a metal powder containing iron provided
on a surface thereof with a glassy thin film, the method
comprising: converting a solution into microfine droplets, wherein
the solution contains a thermally decomposable metal compound
comprising an iron compound and a glass precursor that produces a
glassy substance that does not form a solid solution with a metal
produced from the thermally decomposable metal compound by thermal
decomposition; and heating the droplets, while the droplets are
dispersed in a carrier gas, at a temperature higher than a
decomposition temperature of the thermally decomposable metal
compound, higher than a decomposition temperature of the glass
precursor, and higher than a melting point of the metal produced
from the thermally decomposable metal compound, to produce the
metal powder comprising the metal and to produce a glassy substance
in a vicinity of the surface of the metal powder, wherein the
glassy substance contains at least 40 mass % of SiO2 in terms of
oxide, and the glass precursor is prepared such that a melting
temperature (Tm.sub.M) of the metal and a liquid phase temperature
(Tm.sub.G) of a mixed oxide of the glassy substance satisfy the
following formula (1): -100[.degree.
C.].ltoreq.(Tm.sub.M-Tm.sub.G).ltoreq.500[.degree. C.] (1).
2. The method according to claim 1, wherein the melting temperature
Tm.sub.M and the liquid phase temperature Tm.sub.G satisfy the
following formula (2): -50[.degree.
C.].ltoreq.(Tm.sub.M-Tm.sub.G).ltoreq.300[.degree. C.] (2).
3. The method according to claim 1, wherein the melting temperature
Tm.sub.M and the liquid phase temperature Tm.sub.G are both at
least 1100.degree. C.
4. The method according to claim 1, wherein the solution contains 5
to 30 mass %, as the mass % with respect to the overall solution,
of a reducing agent that is soluble in the solution and exhibits a
reducing activity during the aforementioned heating.
5. The method according to claim 4, wherein the reducing agent
comprises at least one selected from the group consisting of
methanol, ethanol, propanol, ethylene glycol, propylene glycol,
diethylene glycol, and tetraethylene glycol.
6. The method according to claim 1, wherein the total content in
the solution of the thermally decomposable metal compound
comprising the iron compound and the glass precursor is 20 to 100
g/L as a total concentration of the two components as an amount of
metal components produced from the thermally decomposable metal
compound by thermal decomposition and an amount of glass components
in terms of oxide produced from the glass precursor by thermal
decomposition.
7. (canceled)
8. The method according claim 1, wherein the metal comprises nickel
and iron.
9. The method according to claim 8, wherein a mass ratio between
the nickel and iron is nickel:iron=40:60 to 85:15.
10. The method according to claim 1, wherein an iron component
originating from the iron compound is present in the glassy thin
film.
11. (canceled)
12. The method according to claim 1, wherein the glassy substance
contains at least one selected from MgO, CaO, SrO, and BaO in terms
of oxide.
13. The method according to claim 1, wherein 1 to 20 volume % of a
reducing gas is present in the carrier gas.
14. The method according to claim 13, wherein the reducing gas is
at least one selected from hydrogen, carbon monoxide, methane, and
ammonia gas.
15. The method according to claim 2, wherein the melting
temperature Tm.sub.M and the liquid phase temperature Tm.sub.G are
both at least 1100.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing a
metal powder that is coated with a glassy thin film.
BACKGROUND ART
[0002] Mobile devices, e.g., notebook computers, smart phones, and
so forth, have in recent years undergone substantial reductions in
size and weight and a substantial increase in performance.
Increasing the frequency of the switched-mode power supply is
essential for reducing the size and boosting the performance of
these mobile devices, and in association with this the drive
frequencies of various magnetic elements, e.g., the choke coil and
inductor, incorporated in the mobile devices must also accommodate
the frequency increases. However, when the drive frequency of a
magnetic element is increased, the problem occurs of an increase in
eddy current-based losses in the magnetic cores incorporated in the
individual magnetic elements.
[0003] The following is therefore done in order to reduce the eddy
current losses in the case of use at high frequencies: coating an
insulating material on the particle surfaces in a soft magnetic
powder to interpose a coating layer of the insulating material
between the individual particles, thereby cutting off, between the
particles, the eddy current produced in the magnetic core.
[0004] For example, PL 1 discloses a soft magnetic powder having a
surface coated with an inorganic insulating layer and a resin
particle layer. This soft magnetic powder is obtained by forming an
inorganic insulating layer including a low-melting-point glass on
the surface of a soft magnetic powder--by the application to a
preliminarily prepared soft magnetic powder of a powder coating
method such as mechanofusion, a wet method such as electroless
plating or a sol-gel procedure, or a dry method such as
sputtering--and subsequent to this mixing a resin powder with the
soft magnetic powder on which the inorganic insulating layer has
been further formed.
[0005] PL 2 discloses a method for producing a composite-coated
soft magnetic powder wherein a coating layer in which boron nitride
predominates is formed, using inexpensive materials, on the surface
of a ferrous soft magnetic powder. Specifically, a mixed powder is
obtained by mixing a preliminarily prepared iron oxide powder,
silicon carbide powder, carbon powder, and borosilicate glass
powder using, for example, a mixer, and this mixed powder is heated
at 1,000 to 1,600.degree. C. in a nitrogen-containing nonoxidizing
atmosphere to form a boron nitride layer produced by the
decomposition of the borosilicate glass and a metal oxide layer on
the surface of an Fe--Si alloy powder.
[0006] However, in the preliminary preparation of the soft magnetic
powder in the methods of PL 1 and PL 2 for producing a coated soft
magnetic powder, the particle diameter and/or particle size
distribution of the preliminarily prepared soft magnetic powder
must be adjusted into a suitable range depending on the
circumstances. In addition, the composition of the insulator that
will be coated, as well as the amount of coating, must be
controlled in the coating step for forming the insulating layer on
the surface. As a consequence, it has been all but impossible to
form a uniform and homogeneous insulating layer on the surface of a
soft magnetic powder.
[0007] As described in PL 3 and PL 4, soft magnetic powders as such
have generally been prepared by a heretofore known gas atomization
method, mechanical pulverization method, or gas phase reduction
method.
[0008] On the other hand, spray pyrolysis is known as a method for
producing the metal powders used mainly in conductive pastes.
[0009] PL 5, PL 6, and PL 7 disclose art in which a solution
containing one or two or more thermally decomposable metal
compounds is sprayed to convert the solution into microfine
droplets and these droplets are heated to a temperature higher than
the decomposition temperature of the metal compound, or are heated
desirably at around or above the melting point of the metal, to
thermally decompose the metal compound and produce metal particles.
These spray pyrolysis methods can produce a metal powder that
exhibits a good crystallinity, a high density, and a high
dispersion performance and also support facile control of the
particle diameter. In addition, spray pyrolysis offers the
important advantage of enabling the formation of a coating layer on
the metal powder surface at the same time as production of the
metal powder; this is achieved by the addition, to the metal
compound solution that is the starting material for the target
metal powder, of a precursor for, e.g., a metal or semimetal poorly
solid-soluble in the metal powder or the oxide of such a metal or
semimetal. This is thought to occur as follows: since the metal
powder yielded by spray pyrolysis has a good crystallinity and few
defects in the particle interior and is almost entirely free of
grain boundaries, the coating material produced by thermal
decomposition is inhibited from being produced in the interior of
the metal powder and is forced out to the particle surface, thereby
produced at high concentrations in the vicinity of the surface. In
addition, since the composition of the product basically conforms
to the composition of the metal compound in the solution, it is
also easy to control the composition of not only the metal powder
but also the coating layer.
[0010] For these reasons, metal particles having a coating layer on
the surface can be produced by spray pyrolysis without requiring a
separate coating step. For example, PL 8, filed by the present
applicant, describes an invention in which a metal powder having a
glassy thin film coated on at least a portion of the surface is
produced by spray pyrolysis without the introduction of a separate
coating step.
CITATION LIST
Patent Literature
[0011] PL 1: WO 2005/015581 (Japanese Patent No. 4452240)
[0012] PL 2: Japanese Patent Application Laid-open No.
2014-192454
[0013] PL 3: Japanese Patent Application Laid-open No.
H09-256005
[0014] PL 4: Japanese Patent Application Laid-open No.
2003-49203
[0015] PL 5: Japanese Examined Patent Publication No. S63-31522
[0016] PL 6: Japanese Patent Application Laid-open No.
H06-172802
[0017] PL 7: Japanese Patent Application Laid-open No.
H06-279816
[0018] PL 8: Japanese Patent Application Laid-open No. H10-330802
(Japanese Patent No. 3206496)
SUMMARY OF INVENTION
Technical Problem
[0019] The metal powder described in PL 8 is used mainly in
conductive pastes for forming a conductor layer in laminated
ceramic electronic components and in particular is a metal powder
having a surface coated with a glassy thin film for the purpose of
improving the oxidation resistance of the metal powder during
firing of the conductive paste. As a consequence, as long as an
amount effective for this purpose is attached, there is no need for
the glassy thin film to coat the entire metal powder surface and
the coating of at least a portion of the metal surface is
sufficient.
[0020] According to investigations by the present inventors, the
production method described in PL 8 can produce a wide variety of
glassy thin film-coated metal powders using a large number of glass
composition/metal species combinations. On the other hand, there
are instances with this method where it is not necessarily easy to
obtain a metal powder in which the surface is uniformly coated with
a glassy thin film, and with at least some metal species it has not
been possible to carry out metal particle production or a uniform
coating of the metal particle surface with a glassy thin film and a
tendency has been seen for the glassy thin film to be locally
deposited only on a specific portion of the metal powder surface.
In such cases, improvements are obtained as various control
parameters, i.e., the furnace heating temperature and atmosphere
and the cooling conditions, are more strictly controlled, but it is
more difficult to strictly control the control parameters as the
number of parameters to be controlled increases.
[0021] According to investigations by the present inventors, the
trends described above were observed to a pronounced degree in
particular when the metal powder was a soft magnetic powder
containing iron (Fe).
[0022] With respect to spray pyrolysis, an object of the present
invention is therefore to provide a production method that,
regardless of the metal species, readily yields a metal powder that
has a uniform and homogeneous glassy thin film over the entire
surface without local deposition of the glassy thin film on a
specific portion of the metal powder surface.
Solution to Problem
[0023] The present invention, which addresses the aforementioned
problem, is a method for producing a metal powder provided on the
surface thereof with a glassy thin film, wherein a solution that
contains a thermally decomposable metal compound and a glass
precursor that produces a glassy substance that does not form a
solid solution with the metal produced from the metal compound by
thermal decomposition is converted into microfine droplets, and the
droplets are heated, while they are dispersed in a carrier gas, at
a temperature higher than the decomposition temperature of the
metal compound, higher than the decomposition temperature of the
glass precursor, and higher than the melting point of the metal
produced from the metal compound, to produce a metal powder
containing the metal and produce a glassy substance in the vicinity
of the surface of the metal powder,
[0024] wherein the glass precursor is prepared such that the
melting temperature Tm.sub.M of the metal and the liquid phase
temperature Tm.sub.G of the mixed oxide of the glassy substance
satisfy the following formula (1).
-100[.degree. C.].ltoreq.(Tm.sub.M-Tm.sub.G).ltoreq.500[.degree.
C.] (1)
Advantageous Effects of Invention
[0025] In accordance with the present invention, a metal powder
having a glassy thin film with a uniform film thickness and a glass
composition and so forth that is homogeneous can be relatively
easily obtained without strict control of a large number of complex
control parameters.
BRIEF DESCRIPTION OF DRAWINGS
[0026] FIG. 1 is a transmission electron microscope (TEM) image
that shows an image of an entire particle in a metal powder
provided with a glassy thin film on the surface in accordance with
the present invention.
[0027] FIG. 2 is a TEM image that shows a portion of the particle
in FIG. 1.
[0028] FIG. 3 shows the results of line analysis for the particle
in FIG. 2.
[0029] FIG. 4 is a TEM image that shows a portion of the particle
in FIG. 1.
[0030] FIG. 5 shows the results of element mapping on FIG. 4 for
nickel.
[0031] FIG. 6 shows the results of element mapping on FIG. 4 for
iron.
[0032] FIG. 7 shows the results of element mapping on FIG. 4 for
barium.
[0033] FIG. 8 shows the results of element mapping on FIG. 4 for
silicon.
[0034] FIG. 9 shows the results of element mapping on FIG. 4 for
oxygen.
[0035] FIG. 10 is a TEM image that shows a particle surface
according to Experimental Example 17.
[0036] FIG. 11 is an equilibrium phase diagram (as mass %) for
BaO--CaO--SiO.sub.2 glass, as an example of an equilibrium phase
diagram.
DESCRIPTION OF EMBODIMENTS
[0037] The reason is unclear as to why, in the spray pyrolysis
method described in PL 8, a tendency is observed for some glass
composition/metal species combinations wherein the glassy thin film
is prone to be locally deposited only on a specific portion of the
metal powder surface. However, this tendency was strongly observed
when in particular the metal powder is a soft magnetic powder that
contains iron (Fe). The present inventors carried out a variety of
additional tests and hypothesized that the following, for example,
could be contributing factors: generally, many metals including
iron have high melting points; the iron-containing compounds used
as a starting material include many compounds that are resistant to
reduction; and many metals including iron exhibit a relatively poor
wettability with glass. The present invention was achieved as a
result of intensive research based on these hypotheses.
[0038] [Metal Powder]
[0039] There are no particular limitations on the metal powder in
the present invention, and the metal powder in the present
invention encompasses the powder of a single metal and the powder
of an alloy. However, the operation and effect of the present
invention accrue to a greater degree in the case of the production
of a metal powder having a relatively high melting point. The
melting point (Tm.sub.M) of the metal is thus preferably at least
900.degree. C. and is particularly preferably at least 1100.degree.
C.
[0040] The metal preferably contains iron and is particularly
preferably a nickel-iron alloy containing nickel and iron. The
nickel and iron contents are not particularly limited, but the mass
ratio between the nickel and iron is preferably in the range
nickel:iron=40:60 to 85:15, whereamong a permalloy (nickel-iron
alloy with a nickel content of around 78.5 mass %) provides a high
magnetic permeability and is thus advantageous for the present
invention.
[0041] Unless specifically indicated otherwise, in the present
Specification a numerical value range that is given using "to"
indicates a range that includes the numerical values given before
and after the "to". In addition, "major component" refers to a
component for which the content exceeds 50 mass %.
[0042] The nickel-iron alloy may also contain a metal such as
molybdenum, copper, chromium, and so forth.
[0043] There are no particular limitations on the particle diameter
of the metal powder, but the average particle diameter is
preferably approximately 0.2 to 20 .mu.m.
[0044] [Glassy thin Film]
[0045] The glassy substance (also referred to simply as glass)
constituting the glassy thin film may be amorphous or may contain
crystals in an amorphous film, but the difference
(=Tm.sub.M-Tm.sub.G) between the melting point (Tm.sub.M) of the
metal and the liquid phase temperature (Tm.sub.G), where the
components of the glass are considered as a mixture of the oxides
(referred to here as the "mixed oxide"), is preferably in the range
from -100.degree. C. to 500.degree. C. The present invention thus
preferably satisfies the following formula (1).
-100[.degree. C.].ltoreq.(Tm.sub.M-Tm.sub.G).ltoreq.500[.degree.
C.] (1)
[0046] Coating of the entire metal powder surface with a glassy
thin film is readily achieved when the melting point Tm.sub.M of
the metal and the liquid phase temperature Tm.sub.G satisfy the
aforementioned condition.
[0047] When the value of (Tm.sub.M-Tm.sub.G) is lower than
-100.degree. C., it is difficult for vitrification from the glass
starting material (glass precursor) to occur; when greater than
500.degree. C., the fluidity of the produced glass is too high and
as a consequence the occurrence of segregation of the glass on the
metal powder surface, partial exposure of the surface, and so
forth, is facilitated. In either case, coating of the entire
surface of the metal powder with a glassy thin film becomes
difficult.
[0048] (Tm.sub.M-Tm.sub.G) is more preferably in the range from -80
to 400.degree. C. and is particularly preferably in the range from
-50 to 300.degree. C. The present invention thus particularly
preferably satisfies the following formula (2).
-50[.degree. C.].ltoreq.(Tm.sub.M-Tm.sub.G).ltoreq.300[.degree. C.]
(2)
[0049] The liquid phase temperature Tm.sub.G is influenced by the
composition of the glassy substance. Thus, in the present
invention, the glass starting material (glass precursor) is
prepared by determining a glass composition so that the
aforementioned condition with respect to the melting point Tm.sub.M
of the target metal is satisfied.
[0050] According to investigations by the present inventors, for
iron-containing metal powders the aforementioned condition with
respect to Tm.sub.M and Tm.sub.G can be readily satisfied by using
a silicate based glass. The use of a silicate based glass that
provides an SiO.sub.2 content in the glassy thin film of at least
40 mass % in terms of oxide is particularly favorable for the
present invention. Tm.sub.G is preferably at least 900.degree. C.
and is particularly preferably at least 1100.degree. C., although
this will also vary as a function of the melting temperature
Tm.sub.M of the metal.
[0051] The silicate based glass preferably contains an
alkaline-earth metal and specifically preferably contains at least
one selected from the group consisting of MgO, CaO, SrO, and BaO in
terms of oxide. The alkaline-earth metal content is particularly
preferably at least 20 mass % in terms of oxide.
[0052] The liquid phase temperature Tm.sub.G in the present
invention can be determined from an equilibrium phase diagram, such
as the one shown in FIG. 11 as an example. As necessary, it may
otherwise also be determined from the heat absorption behavior in
differential thermal analysis (DTA) or differential scanning
calorimetry (DSC).
[0053] When iron is present in the metal powder in the production
method according to the present invention, as indicated below the
presence of an iron component in the glassy thin film on the metal
powder surface can then also be confirmed. Since a ferrous compound
is not used in the glass starting material (precursor), it is
thought that the iron component in this glass originates from the
iron compound present in the metal compound used as a starting
material for the metal powder and diffuses into the glass during
heating. In addition, the present inventors hypothesize that the
wettability between the glass and the iron component in the metal
powder is improved by the presence of the iron component in the
glass, which as a result enables the formation of a strong glass
coating film even on an iron-containing metal powder.
[0054] [Spray Pyrolysis]
[0055] The metal powder according to the present invention is
produced by spray pyrolysis. In specific terms, the present
invention is a method for producing a metal powder provided on the
surface thereof with a glassy thin film, wherein a solution that
contains a thermally decomposable metal compound and a glass
precursor that produces a glassy substance that does not form a
solid solution with the metal produced from the metal compound by
thermal decomposition is converted into microfine droplets, and the
droplets are heated, while they are dispersed in a carrier gas, at
a temperature higher than the decomposition temperature of the
metal compound, higher than the decomposition temperature of the
glass precursor, and higher than the melting point of the metal
produced from the metal compound, to produce a metal powder
containing the metal and to produce a glassy substance in the
vicinity of the surface of the metal powder.
[0056] A complex salt or double salt or one or two or more
thermally decomposable salts, e.g., a nitrate salt, sulfate salt,
chloride, ammonium salt, phosphate salt, carboxylate salt, or resin
acid salt of a metal or a metal alcoholate may be used in the
present invention as the thermally decomposable metal compound that
is the starting compound for the metal particles. When a mixture of
two or more metal salts is used, an alloy particle or mixed
particle of two or more metals can then be obtained. One or two or
more glass-forming glass precursors are added to a solution of this
major component metal compound dissolved in water, an organic
solvent such as acetone or an ether, or a mixed solvent of the
preceding.
[0057] There are no limitations on the glass precursor other than
that, under the metal particle production conditions according to
the present method, the glass precursor should undergo
vitrification and the oxide (glass) produced by thermal
decomposition should not go into solid solution in the metal
particle. A suitable selection from, for example, the following can
be used as the glass precursor: boric acid, silicic acid, and
phosphoric acid; thermally decomposable salts, e.g., various
borates, silicates, and phosphates as well as the nitrates,
sulfates, chlorides, ammonium salts, phosphate salts, carboxylate
salts, alcoholates, and resin acid salts of various metals; double
salts; and complex salts.
[0058] In the present invention, the mixed solution of the metal
compound and glass precursor is converted into microfine droplets
using a spray device, e.g., an ultrasound type or a dual-flow
nozzle type, and this is followed by thermal decomposition by
heating to a temperature higher than the decomposition temperature
of the metal compound and the decomposition temperature of the
glass precursor. When a mixture of two or more compounds is used
for the metal compound, heating is carried out at a temperature
higher than the decomposition temperature of the metal compound
having the highest decomposition temperature.
[0059] The heat treatment in the present invention is carried out
at a high temperature at or above the melting point of the majority
component metal. While the effect of forcing out the glass
component can be obtained even at a heating temperature lower than
the melting point, in such cases a metal powder having a good
crystallinity is not obtained, and in addition the metal powder has
an irregular shape, which may lead to an inadequate densification
and dispersibility.
[0060] The atmosphere during heating is selected as appropriate
from oxidizing atmospheres, reducing atmospheres, and inert
atmospheres in conformity with, for example, the species of metal
compound, the species of glass precursor, the heating temperature,
and so forth, but is particularly preferably a reducing atmosphere
when a metal powder is being produced for which a base metal is the
major component of the metal. In such a case, the addition to the
solution is preferably made in advance of a reducing agent that is
soluble in the solution and that does not exhibit a reducing
activity in the absence of heating (for example, during preparation
of the spray solution) and exhibits a reducing activity only during
heating. The reducing agent can be exemplified by at least one
selected from the group consisting of methanol, ethanol, propanol,
ethylene glycol, propylene glycol, diethylene glycol, and
tetraethylene glycol. The base metal is not particularly limited,
but iron, cobalt, nickel, copper, and so forth are preferred and
iron, nickel, and alloys containing them are particularly preferred
in the present invention.
[0061] While this will depend on the species of metal compound
used, the amount of the reducing agent added to the solution is
preferably 5 to 30 mass % of the whole of the solution.
[0062] While larger amounts of reducing agent are advantageous for
reducing the metal compound, this causes an increase in the
concentration of the solution and thus impedes spraying in the case
of spray pyrolysis. When the amount of reducing agent added to the
solution is in the range indicated above, much of the metal
compound can be reduced even in the case of use of a metal compound
resistant to reduction, while in addition there are no impediments
to spraying of the solution.
[0063] In addition to the use of the aforementioned reducing agent,
in the present invention it is preferable that a reducing gas is
optionally present in the range of 1 to 20 volume % in the carrier
gas that transports the microfine droplets. The reducing gas can be
exemplified by at least one selected from the group consisting of
hydrogen, carbon monoxide, methane, and ammonia gas. Through the
incorporation of a reducing gas in the carrier gas in combination
with the incorporation of a reducing agent in the solution,
particularly even in the case of the use of a metal compound
resistant to reduction, spray pyrolysis can be carried out while
easily controlling reduction without having to increase the amount
of reducing agent in the solution and thus without causing
impediments to spraying of the solution.
[0064] The present invention, because it produces a metal powder by
spray pyrolysis from a mixed solution starting material, can yield
a target metal powder having a glassy thin film on the surface
through the selection of the composition of the individual
components, i.e., the thermally decomposable metal compound and the
glass precursor, and the amount of addition of the glass precursor
relative to the metal compound. The total content of the thermally
decomposable metal compound and the glass precursor in the mixed
solution is less than 500 g/L as the total concentration in the
mixed solution of the two components as the amount of metal
components produced from the metal compound by thermal
decomposition and the amount of glass components in terms of oxide
produced from the glass precursor by thermal decomposition. This
total content is advantageously 20 to 100 g/L from the standpoint
of the ease of control. When a metal powder particle containing two
or more metals is produced using a metal compound that contains two
or more metals or using two or more metal compounds, the
aforementioned amount of metal components is then the total amount
of metal components produced from the metal compound(s) by thermal
decomposition. The mixing ratio between the metal compound and
glass precursor in the mixed solution is determined by the mass
ratio of the amount of glass components in terms of oxide relative
to the amount of metal components that will be provided by spray
pyrolysis. No effect occurs when the amount of glass components in
terms of oxide produced from the glass precursor relative to the
amount of metal components produced from the metal compound is
smaller than 0.1 mass %. When, on the other hand, the amount of
glass precursor addition is too large, the glass produced from the
glass precursor is produced segregated to only a portion of the
metal particle surface and the uniform coating of the entire
particle surface with the glassy thin film becomes difficult. Thus,
while this also depends on the density of the produced glass, from
a practical perspective the glass precursor is added so as to
provide 0.1 to 20 mass % as the aforementioned amount of glass
components in terms of oxide relative to the aforementioned amount
of metal components, while an addition that provides 0.5 to 15 mass
% is particularly desirable. The production method according to the
present invention makes it possible to easily obtain metal powder
particles that are uniformly coated over the entire surface with a
homogeneous glassy thin film; however, the production may also
occur to a very small extent of metal powder particles that are
provided with a glassy thin film that is slightly nonuniform to a
degree that is not problematic at a practical level. The metal
powder provided by the production according to the present
invention does not exclude such a powder that is not problematic at
a practical level.
[0065] The present invention is specifically described below using
examples, but the present invention is not limited to or by these
examples.
EXAMPLES
Experimental Example 1
[0066] Nickel nitrate hexahydrate and iron nitrate were weighed out
so as to provide the metal shown in Table 1 and were dissolved in
water to provide the metal component concentration in the solution
also shown in the same table. The following were added to this with
mixing to produce a starting solution: ethylene glycol (MEG) as a
reducing agent and tetraethyl orthosilicate (TEOS) and barium
nitrate that had been weighed out to provide the glass component
shown in Table 1 [The numerical values for the glass composition in
the table give the content proportion in mass % with respect to the
total mass when converted to the oxide. In addition, the amount of
added glass components in the table is the amount (mass %) of glass
components in terms of oxide with respect to the amount of the
metal components; this also applies to Tables 2 and 3.]. The metal
component concentration (g/L) in the solution shown in Table 1 and
Tables 2 and 3 is the metal compound content per 1 L of solution,
as the metal components produced from the metal compound by thermal
decomposition. In addition, the amount of reducing agent in the
solution given in Table 1 and Tables 2 and 3 is the content (mass
%) of the reducing agent with respect to the solution as a
whole.
[0067] The starting solution was converted into microfine droplets
using an ultrasound spray device and, using nitrogen gas as the
carrier at the flow rate given in Table 1, was fed into a ceramic
tube heated to 1550.degree. C. in an electric furnace. The droplets
were thermally decomposed while passing through a heating zone and
were collected in the form of a powder.
[0068] According to the results of X-ray diffraction, the collected
powder was a nickel-iron alloy powder, and diffraction lines other
than this were not detected. When this powder was washed with 5%
dilute hydrochloric acid, the amount of added material in the
powder after washing was substantially depleted while there was
almost no dissolution of the nickel or iron.
[0069] FIG. 1 is a TEM image that shows an image of an entire
particle in the powder immediately after collection. FIG. 3 gives
the results for line analysis of this powder in the direction of
the arrow in FIG. 2 using energy-dispersive X-ray analysis (EDX).
While powder with a small particle diameter is seen in FIG. 1, a
powder with a more uniform particle diameter can be obtained as
necessary by carrying out a classification process thereon.
[0070] FIGS. 5 to 9 give the mapping results for each of the
elements nickel, iron, barium, silicon, and oxygen, respectively,
from the TEM image of the powder given in FIG. 4. These analyses
demonstrated for this powder that silicon and barium were produced
at high concentrations on the surface of a nickel-iron alloy powder
and were present in the state of BaO--SiO.sub.2 glass that is
homogeneous and X-ray amorphous. As shown in FIG. 6, the presence
of iron in the glassy thin film on the surface of the nickel-iron
alloy powder could be confirmed.
[0071] The following are given in Table 1: the melting point
(Tm.sub.M) of the alloy, the liquid phase temperature (Tm.sub.G)
determined from the equilibrium phase diagram for the mixed oxide
for the glass component, the glass coating ratio [5] with respect
to the particle surface as determined from the area by element
mapping, and the thickness [nm] of the glassy thin film as
determined from the TEM image.
TABLE-US-00001 TABLE 1 metal liquid metal melting phase component
point temperature experimental concentration Tm.sub.M Tm.sub.G
example metal [g/L] alloy ratio [.degree. C.] glass component
[.degree. C.] Tm.sub.M - Tm.sub.G 1 Ni/Fe 35 78.5/21.5 1450
33BaO--67SiO.sub.2 1490 -40 2 Ni/Fe 35 78.5/21.5 1450
47BaO--53SiO.sub.2 1400 50 3 Ni/Fe 35 79.5/20.5 1451
38BaO--14CaO--48SiO.sub.2 1190 261 4 Ni/Fe 35 78.5/21.5 1450
38BaO--14CaO--48SiO.sub.2 1190 260 5 Ni/Fe 35 78.5/21.5 1450
38BaO--14CaO--48SiO.sub.2 1190 260 6 Ni/Fe 35 78.5/21.5 1450
38BaO--14CaO--48SiO.sub.2 1190 260 7 Ni/Fe 35 78.5/21.5 1450
38BaO--14CaO--48SiO.sub.2 1190 260 8 Ni/Fe 35 78.5/21.5 1450
38BaO--14CaO--48SiO.sub.2 1190 260 9 Ni/Fe 35 78.5/21.5 1450
38BaO--14CaO--48SiO.sub.2 1190 260 10 Ni/Fe 35 78.5/21.5 1450
38BaO--14CaO--48SiO.sub.2 1190 260 11 Ni/Fe 35 77.5/22.5 1449
38BaO--14CaO--48SiO.sub.2 1190 259 12 Ni/Fe 35 76.5/23.5 1447
38BaO--14CaO--48SiO.sub.2 1190 257 13 Ni/Fe 35 90/10 1452
38BaO--14CaO--48SiO.sub.2 1190 262 14 Ni/Fe 35 85/15 1451
38BaO--14CaO--48SiO.sub.2 1190 261 15 Ni/Fe 35 45/55 1445
38BaO--14CaO--48SiO.sub.2 1190 255 16 Ni/Fe 35 78.5/21.5 1450
44BaO--6CaO--51SiO.sub.2 1300 150 17 Ni/Fe 35 76.5/23.5 1447
66Bi.sub.2O.sub.3--34MnO 830 617 amount of amount of addition of
reducing amount of thickness glass reducing agent reducing coating
of glassy experimental components agent in solution carrier gas
(N.sub.2) agent in ratio thin film example [mass %] in solution
[mass %] flow rate/minute carrier gas [%] [nm] 1 2 MEG 20 80 L/min
-- 100 5 2 2 MEG 20 80 L/min -- 100 5 3 2 MEG 20 80 L/min -- 100 5
4 0.2 MEG 20 80 L/min -- 100 1 5 0.5 MEG 20 80 L/min -- 100 1.5 6 1
MEG 20 80 L/min -- 100 2 7 2 MEG 20 80 L/min -- 100 5 8 3 MEG 20 80
L/min -- 100 7 9 5 MEG 20 80 L/min -- 100 9 10 10 MEG 20 80 L/min
-- 100 12 11 2 MEG 20 80 L/min -- 100 5 12 2 MEG 20 80 L/min -- 100
5 13 2 MEG 20 80 L/min -- 100 5 14 2 MEG 20 80 L/min -- 100 5 15 2
MEG 20 80 L/min -- 100 5 16 2 MEG 20 80 L/min -- 100 5 17 2 MEG 10
80 L/min -- not greater not than 50 measured
Experimental Example 2
[0072] A nickel-iron alloy powder coated with a BaO--SiO.sub.2
glassy thin film was obtained as in Experimental Example 1 except
that the glass components were as described in Table 1. The
analytic results, obtained as in Experimental Example 1, are given
in Table 1.
Experimental Examples 3 to 17
[0073] Nickel-iron alloy powders coated with a glassy thin film
were obtained as in Experimental Examples 1 and 2 except that for
each experimental example the metal composition, glass components,
amount of added glass components, and amount of reducing agent
added to the solution [content (mass %) of the reducing agent with
respect to the entire solution] are set as indicated in Table 1.
Calcium nitrate was used as the calcium source for the glass
components; manganese nitrate was used as the manganese source; and
bismuth citrate was used as the bismuth source. The analytic
results, obtained as in Experimental Example 1, are given in Table
1.
[0074] As shown in FIG. 10 for Experimental Example 17, a situation
was observed in which the glassy thin film was produced segregated
to only a portion of the metal powder surface, and for this reason
the thickness of the glassy thin film was not measured. It is
hypothesized that this result occurred due to the large difference
between the melting point Tm.sub.M and the liquid phase temperature
Tm.sub.G in Experimental Example 17.
Experimental Examples 18 to 21
[0075] Iron powders coated with a glassy thin film were obtained in
each of these experimental examples as in Experimental Example 1
except that iron nitrate was used for the metal components, that
the procedure was carried out so as to provide the metal components
concentration in the solution and the glass components as given in
Table 2, and that the reducing agent given in Table 2 was added to
the carrier gas. The amount of reducing agent in the solution is,
as above, the content (mass %) of the reducing agent with respect
to the entire solution. In addition, hydrogen gas and carbon
monoxide were added in the amounts (volume %) shown in Table 2 to
the nitrogen gas used as a carrier gas. The analytic results,
obtained as in Experimental Example 1, are given in Table 2.
[0076] A region in which the thickness of the glassy thin film was
not uniform was seen on the surface to a very slight degree for the
iron powder of Experimental Example 19, but this was still usable
at a practical level.
TABLE-US-00002 TABLE 2 metal liquid metal melting phase component
point temperature experimental concentration Tm.sub.M Tm.sub.G
example metal [g/L] alloy ratio [.degree. C.] glass component
[.degree. C.] Tm.sub.M - Tm.sub.G 18 Fe 20 -- 1538
33BaO--67SiO.sub.2 1490 48 19 Fe 20 -- 1538
38BaO--14CaO--48SiO.sub.2 1190 348 20 Fe 20 -- 1538
38BaO--14CaO--48SiO.sub.2 1190 348 21 Fe 20 -- 1538
38BaO--14CaO--48SiO.sub.2 1190 348 amount of amount of addition of
reducing amount of thickness glass reducing agent reducing coating
of glassy experimental components agent in solution carrier gas
(N.sub.2) agent in ratio thin film example [mass %] in solution
[mass %] flow rate/minute carrier gas [%] [nm] 18 2 MEG 20 80 L/min
4% H.sub.2, 100 5 12.5% CO 19 2 MEG 20 80 L/min 4% H.sub.2, 100 3~5
12.5% CO 20 2 MEG 25 80 L/min 3% H.sub.2 100 5 21 2 MEG 15 80 L/min
5.5% H.sub.2, 100 5 12.5% CO
Experimental Examples 22 to 26
[0077] Metal powders coated with a glassy thin film were obtained
as in Experimental Example 1, except that the metal composition,
the metal component concentration in the solution, the glass
components, and the reducing agent added to the solution [the
amount of reducing agent in the solution is the content (mass %)
with respect to the overall solution] are changed in accordance
with Table 3. Tetraethylene glycol (TEG) was used as the reducing
agent in Experimental Example 22, while, as in Experimental Example
1, MEG was used in Experimental Examples 23 to 25. No reducing
agent was used in Experimental Example 26. The analytic results,
obtained as in Experimental Example 1, are given in Table 3.
TABLE-US-00003 TABLE 3 metal liquid metal melting phase component
point temperature experimental concentration Tm.sub.M Tm.sub.G
example metal [g/L] alloy ratio [.degree. C.] glass component
[.degree. C.] 22 Ni 40 -- 1455
35.2BaO--14.3CaO--45.7SiO.sub.2--4.8MnO 1145 23 Cu/Ni 60 85/15 1170
35.2BaO--14.3CaO--45.7SiO.sub.2--4.8MnO 1145 24 Cu/Ni 60 90/10 1150
35.2BaO--14.3CaO--45.7SiO.sub.2--4.8MnO 1145 25 Cu 40 -- 1085
35.2BaO--14.3CaO--45.7SiO.sub.2--4.8MnO 1145 26 Ag 40 -- 962
66Bi.sub.2O.sub.3--34MnO 830 amount of amount of addition of
reducing amount of thickness glass reducing agent reducing coating
of glassy experimental components agent in solution carrier gas
(N.sub.2) agent in ratio thin film example Tm.sub.M - Tm.sub.G
[mass %] in solution [mass %] flow rate/minute carrier gas [%] [nm]
22 310 2 TEG 8 80 L/min -- 100 5 23 25 2 MEG 10 80 L/min -- 100 5
24 5 2 MEG 10 80 L/min -- 100 5 25 -60 2 MEG 10 80 L/min -- 100 5
26 132 2 -- -- 80 L/min -- 100 5
* * * * *